156 TRANSACTIONS OF THE 
arranged that air can be blown through the charge. This 
air oxidizes the impurities and this rapid oxidization 
‘maintains the high temperatures. After the impurities 
have been removed the proper amounts of carbon, sili- 
con, manganese, chromium, etc., are added to make a 
grade of steel, having a definite chemical composition. 
Here we have seen that every step is controlled, 
chemically, and depends upon a chemical reaction. 
One of the products obtained by smelting of copper 
ores in a blast furnace is copper matte, a copper iron 
sulphide which is purified by oxidization of the sulphur 
and iron. This oxidization process is carried out in a 
convertor similar to the one used in the manufacture of 
steel from pig iron but in this case the sulphur burns to 
sulphur dioxide and the iron oxide combines with the 
silica in the lining of convertor and forms a slag which 
is poured off. This purification is based upon the chemi- 
cal fact that oxygen and sulphur will combine at increased 
temperatures to form sulphur dioxide and at the same 
time furnish sufficient heat to keep the charge melted. 
All roasting operations depend upon the chemical 
reaction between oxygen and sulphur at increased tem- 
peratures. There are many types of roasting furnaces 
in use but they all depend upon the same chemical re- 
action for their successful operation. 
In the metallurgy of zine we smelt the oxidized 
material, either a natural product or one obtained by 
roasting a sulphide ore, in a closed fire clay retort in 
which the ore is mixed with a large excess of carbon. 
Zine oxide is reduced to metallic zinc at about 1030° 
C and since the boiling point of zinc is 930° C the reduced 
zinc vapors pass off and are condensed as the liquid 
metal. By increasing the temperature of the retorts to 
1200-1300° C it is possible to increase the capacity of the 
retorts. Zine vapors condense to liquid metal in the 
range of temperatures between 420-860° C so in practice 
the temperature of the condensors is kept about 500° C 
so that the zinc can be tapped off in the liquid state. 
These peculiar chemical properties of zinc form the 
basis for this means of extracting the metal from its 
ores. 
The separation of silver from lead in lead silver 
